Interior or containing surfaces of water storage tanks are generally coated with an electrically resistant material, typically epoxies, vinyls, chlorinated rubbers, coal tar enamels, and the like, to retard or prevent corrosion of the tank metal, typically steel, but not limited thereto. The tank is provided with an inlet-outlet pipe, usually disposed at the floor of the tank, or the tank may be provided with separate inlet and outlet pipes. The interior surface of the pipe is uncoated. The tank coatings, even though carefully applied to provide a well coated surface, contain pores and minute flaws allowing the corroding electrolyte occasional contact with very small areas of the underlying metal surfaces.
The water in the tank is relatively quiescent. The water in the throat of the bare inlet-outlet pipe however is frequently flowing at a high velocity, thus leading to the formation of a differential aeration galvanic corrosion cell between the bare pipe (cathodic) and any metal exposed to the quiescent water at a pore or minute coating flaw (anodic) in the submerged surfaces. The differential aeration galvanic corrosion cell promotes corrosion at these pores and minute flaws which accelerates coating breakdown and further coating failure. The larger cathodic area of the uncoated or bare inlet-outlet pipe, metallically coupled to the relatively small total anodic area of the metal exposed at the pores and minute flaws of the coated surfaces, intensifies the corrosion of the metal at these pores and minute flaws.
At the present time, cathodic protection current is applied to prevent corrosion of well coated tanks by either of two known methods. In one, the protective current is applied from an anode, or anodes suspended in the corroding electrolyte, typically water, with manually or automatically regulated applied current to achieve the desired tank-to-water potential with respect to a reference electrode positioned on the coated tank surface below the surface of the water. The tank-to-water potential however rapidly becomes less electronegative with respect to the positioned reference electrode as the bare area of the inlet-outlet pipe is approached due to potential or IR drop of the applied current as it enters the constricted water conductive path to the gross bare area. Thus, potentials regulated to provide adequate corrosion protection and yet not deteriorate the coating over most of the tank surface due to an excessively high electronegative potential will now always provide the necessary protection to the coated surfaces and flaws thereunder approaching the bare inlet-outlet pipe. To clarity, a negative (cathodic) tank-to-water potential of at least 0.85 volts, as measured between the coated tank surface, typically steel, and a saturated copper-copper sulfate reference electrode placed in the water adjacent the coated steel surface, is desired in order to achieve good corrosion control at the coating flaws and to obtain maximum coating life. Thus, a higher protective current must be applied to compensate for the aforementioned IR drop which causes the electronegative potential to exceed 1.10 volts, resulting in coating deterioration by electro-endosmotic effects and disbonding by alkali attack.
In a second method, the protective current, applied from the suspended anode or anodes (vertically hung or ring anodes) is regulated to maintain a selected polarized potential free of IR drop. The polarized potential measured when the protective current is momentarily interrupted eliminates the IR drop caused by the applied current flow through the electrolyte from points on the coated surface to the bare areas to which the protective current flows. Similarly, the "null" bridge circuit method, described in U.S. Pat. No. 3,425,921 to Sudrabin, a co-inventor named herein, eliminates the abovediscussed IR drop. The uncoated throat of the inlet-outlet pipe presents the dominant bare area in the system. The resistance of this bare area surface to the electrolyte is usually many times less than the resistance of the entire area of the pinhole flaws of the well-coated tank. Thus, in accordance with the "law of shunts," most of the protective current will flow onto the bare pipe surface. When the protective current is again applied, the IR drop eliminated in the potential measurement due to its momentary interruption, for example, will actually be included in the voltage measurement between the vessel and the reference electrode positioned in the electrolyte at the coated surface.
In well-coated tanks such as those contemplated for cathodic protection by the apparatus of the present invention, the voltage measured across the coating and the underlying metal often exceeds -2.0 volts when one of the IR drop-free control methods abovementioned was employed, which negative voltage is considerably more negative than the desired or optimum value of -0.85 volts or even the upper limit tolerable value of -1.10 volts.
The present invention substantially overcomes the deficiencies of the abovedescribed methods employed currently to protect well coated tanks and provides apparatus which maintains an optimum protective potential uniformly on all submerged coated surfaces of the tank, the optimum potential being a negative (cathodic) voltage of at least 0.85 volts as measured between the coated steel tank surface and a saturated copper-copper sulfate reference electrode in contact with the electrolyte placed adjacent any point on the submerged coating surface. Cathodic protection is controlled to limit the electronegative protective potential to 1.10 volts to retard coating damage. As well coated tank will include at least one gross bare area such as uncoated inlet-outlet pipe, the gross bare area being considerably greater, say 100 times, than the area of any individual flaw in the coating, the gross bare area, extending two pipe diameters into the bare pipe, being greater than 5 times the total bare area of the randomly located coating pores and flaws.
By practicing the present invention, the electrically resistant coating life is maximized. By positioning the anodes in accordance with the present invention, the anodes will be less subjected to damage by ice formation.